Movement of Fluid Under Pressure.
Force applied to a confined
liquid can cause the liquid to move only when that force exceeds any
other force acting on the liquid in an opposing direction.
Fluid
flow is always in the direction of the lowest pressure.
If the
opposing forces are equal, no movement of fluid takes place.
Fluid under pressure can flow into already filled containers only
if an equal or greater quantity simultaneously flows out of them.
This is an obvious and simple principle, but one that is easily
overlooked.
Effects of Temperature on Liquids.
As in metals, temperature
changes produce changes in the size of a body of liquid.
With the
exception of water, whenever the temperature of a body of liquid
falls, a decrease (contraction) in size of the body of fluid takes
place. The amount of contraction is slight and takes place in direct
proportion to the change in temperature.
When the temperature rises, the body of liquid expands. This is
referred to as "thermal expansion."
The amount of expansion is in
direct proportion to the rise in temperature.
Although the rate of
expansion is relatively small, it is important; some provision is
usually necessary in a hydraulic system to accommodate the increase
in size of the body of liquid when a temperature rise occurs.
MECHANICAL ADVANTAGE
By simple definition, mechanical advantage is equal to the ratio of a
force or resistance overcome by the application of a lesser force or
effort through a simple machine.
This represents a method of
multiplying forces.
In mechanical advantage, the gain in force is
obtained at the expense of a loss in distance.
Discounting
frictional losses, the percentage gain in force equals the percentage
loss in distance.
Two familiar applications of the principles of
mechanical advantage are the lever and the hydraulic jack.
In the
case of the jack, a force of just a pound or two applied to the jack
handle can raise many hundreds of pounds of load. Note, though, that
each time the handle is moved several inches, the load is raised only
a fraction of an inch.
Application in Hydraulics.
The principle used in hydraulics to
develop mechanical advantage is simple.
Essentially it is obtained
by fitting two movable surfaces of different sizes to a confining
vessel, such as pistons within cylinders. The vessel is filled with
fluid, and force (input) is applied to
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